1. Field of the Invention
The present invention relates to an automatic exposure device for a panoramic X-ray photographing apparatus, and more particularly to a panoramic X-ray photographing device for dental diagnosis.
2. Prior Art
The quality of X-ray photographs taken by an X-ray photographing apparatus is determined by whether X-ray tube current is balanced with X-ray tube voltage or not and is judged by the blackening degree (density) of photograph films. Particularly in a panoramic X-ray photographing apparatus for dental diagnosis, the X-ray dose reaching a film surface varies according to the differences between an adult and a child, between a male and a female and between a foretooth and a molar tooth. As a result, the contrast on the film surface differs from place to place such that a good contrast is obtained at some portions, while at other portions a good contrast can not be obtained because of the blackening degree greatly differing from the optimum value. Although this problem can be solved by adjusting X-ray exposure, prior art had the following drawbacks.
That is, the conventional automatic exposure device controls only the tube voltage or tube current according to penetrated X-ray dose. For example, the invention disclosed in Japanese Patent Publication No. 46640/1982 (hereinafter referred to as the former invention) automatically control the X-ray tube voltage of an X-ray generator according to penetrated X-ray dose while the invention disclosed in Japanese Patent Publication No. 12518/1982 (hereinafter referred to as the latter invention) automatically controls the tube current so that a constant ratio is obtained between the penetrated X-ray dose and film speed. In the case of these inventions, either the tube current or voltage, which is not controlled, must be initially set. This initial value must be fixed or manually adjusted by the operator. Therefore, in the case of the former invention, picture quality is determined only by the X-ray intensity. As a result, the contrast is variable and the picture becomes blurred, preventing proper diagnosis. Furthermore, since the tube current to be fixed is apt to be set high, extra X-rays are radiated to patients. Although the latter invention provides a good contrast, it cannot generate picture quality with a proper X-ray intensity based on the actual bone construction of each patient. To solve this problem, another invention has been proposed, which uses a head securing unit to initially set the tube voltage according to the head size of each patient. In this case, however, the initial value is set without fully considering the actual bone construction of each patient, thus problems are caused in actual practice.
In the case of the invention in which feedback control is applied to the tube voltage or current according to the residual penetrated X-ray dose, accurate and stable control is impossible if the actual tube voltage or current fluctuates is inaccurate even when feedback information is correct. If the power voltage fluctuates or the X-ray tube deteriorates (the X-ray tube cannot perform stable operation permanently but deteriorates after use for an extended period) for example, the actual tube voltage and current also fluctuate and the blackening degree cannot be controlled properly even when the feedback applied, preventing generation of proper X-ray pictures having high repeatability.